Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
A client requires power for a new industrial machine with a nameplate rating of 10 kVA, 3-phase, 480V, and a power factor of 0.8 lagging. What is the approximate real power (kW) drawn by this machine?
- A6 kW
- B12 kW
- C10 kW
- D8 kW
Show answer & explanationAnswer & explanation
Correct answer: D. 8 kW
Real power (P) is calculated by multiplying apparent power (S) by the power factor (PF). So, P = S * PF = 10 kVA * 0.8 = 8 kW.
Why the other options are wrong
- A. This would be if the power factor was 0.6 (10 kVA * 0.6 = 6 kW).
- B. This would be if the power factor was 1.2, which is not possible.
- C. 10 kW is the apparent power, not the real power, unless the power factor was 1.0.
Real Power from Apparent Power and Power Factor
Real power (kW) is the actual power consumed by a load that performs useful work, derived by multiplying the apparent power (kVA) by the power factor (PF).
- Real power (P) is measured in watts (W) or kilowatts (kW).
- Apparent power (S) is measured in volt-amperes (VA) or kilovolt-amperes (kVA).
- Power factor (PF) is the ratio of real power to apparent power (PF = P/S).
- PF indicates the efficiency of power utilization; a lower PF means higher reactive power.
Memory trick: P = S x PF, like 'P'eanut 'S'auce 'P'rovider 'F'actory.